In recent years, an antitheft system for use in a vehicle determines whether the vehicle is in a condition of theft based on an inclination angle of the vehicle detected by an angle sensor. That is, the inclination angle is continuously monitored to calculate a change of the inclination angle, and the change of the inclination angle is compared with a predetermined value for detecting the theft of the vehicle. For example, Japanese patent document JP-A-H4-159164 discloses a technique that detects vehicle theft in above-described manner.
The angle sensor outputs an angle signal to an amplifier, and the signal is amplified by the amplifier. The signal from the amplifier passes a filter. Then, the signal is inputted to a microcomputer for processing. In this case, the signal inputted to the microcomputer is affected by a change of the temperature, that is, a signal value suffers from a temperature drift.
The temperature drift that affects the signal value is compensated by using plural calculation units in the disclosure of the above patent document. That is, the calculation units compare the signal value with a predetermined threshold for updating an initial value that is used in an angle calculation at a predetermined interval in respective calculation units. In this manner, accumulation of the temperature drift of the signal value is prevented. In addition, the initial value of the angle calculation is different in each of the calculation units. In this manner, outputs from the calculation units are compensated with each other for detecting the change of the inclination angle that can not be detected by a single calculation unit.
However, even when the initial value of the angle calculation is periodically updated, a sudden change of the temperature in a large amount causes the temperature drift of the signal value that cannot be compensated in terms of preventing a false detection.